Outlook Web Access users see sent items end up in drafts folder

I am having an intermittent problem with one user who connects 100% of the time to outlook web access complain that their sent items are suddenly going into their drafts folder. No other users seems to be experiencing this problem and I am not sure what could be causing it, let alone where to go to start diagnosing such a bug. Has anyone seen anything like this? (The emails are sent out, they just get placed in the wrong folder after)